Method of making amorphous semiconductor alloys and devices using microwave energy

ABSTRACT

A low pressure process for making amorphous semiconductor alloy films and devices at high deposition rates and high gas conversion efficiencies utilizes microwave energy to form a deposition plasma. The alloys exhibit high-quality electronic properties suitable for many applications including photovoltaic and electrophotographic applications. 
     The process includes the steps of providing a source of microwave energy, coupling the microwave energy into a substantially enclosed reaction vessel containing the substrate onto which the amorphous semiconductor film is to be deposited, introducing into the vessel at least one reaction gas and evacuating the vessel to a low enough deposition pressure to deposit the film at high deposition rates with high reaction gas conversion efficiencies without any significant powder or polymeric inclusions. The microwave energy and the reaction gases form a glow discharge plasma within the vessel to deposit an amorphous semiconductor film from the reaction gases onto the substrate. The reaction gases can include silane (SiH 4 ), silicon tetrafluoride (SiF 4 ), silane and silicon tetrafluoride, silane and germane (GeH 4 ), and silicon tetrafluoride and germane. The reaction gases can also include germane or germanium tetrafluoride (GeF 4 ). To all of the foregoing, hydrogen (H 2 ) can also be added. Dopants, either p-type or n-type can also be added to the reaction gases to form p-type or n-type alloy films, respectively. Also, band gap increasing elements such as carbon or nitrogen can be added in the form of, for example, methane or ammonia gas to widen the band gap of the alloys.

RELATED APPLICATION

This is a continuation-in-part of pending application Ser. No. 423,424,filed Sept. 24, 1982, for Method of Making Amorphous SemiconductorAlloys and Devices Using Microwave Energy.

BACKGROUND OF THE INVENTION

This invention relates to an improved method of making amorphoussemiconductor alloys and devices having a substantially increasedreaction gas conversion efficiency and a substantially increaseddeposition rate. The invention more particularly relates to a method ofmaking such alloys and devices by plasma deposition from reaction gaseswherein the plasmas are excited by microwave energy at low pressures.The invention has its most important application in makingphotoresponsive alloys and devices for various photoresponsiveapplications including photoreceptive devices such as solar cells of ap-i-n, p-n, Schottky or MIS (metal-insulator-semiconductor) type;photoconducting medium such as utilized in xerography; photodetectingdevices and photodiodes including large area photodiode arrays.

Silicon is the basis of the huge crystalline semiconductor industry andis the material which has produced expensive high efficiency (18percent) crystalline solar cells for space applications. Whencrystalline semiconductor technology reached a commercial state, itbecame the foundation of the present huge semiconductor devicemanufacturing industry. This was due to the ability of the scientist togrow substantially defect-free germanium and particularly siliconcrystals, and then turn them into extrinsic materials with p-type andn-type conductivity regions therein. This was accomplished by diffusinginto such crystalline material parts per million of donor (n) oracceptor (p) dopant materials introduced as substitutional impuritiesinto the substantially pure crystalline materials, to increase theirelectrical conductivity and to control their being either of a p or nconduction type. The fabrication processes for making p-n junctioncrystals involve extremely complex, time-consuming, and expensiveprocedures. Thus, these crystalline materials useful in solar cells andcurrent control devices are produced under very carefully controlledconditions by growing individual single silicon or germanium crystals,and when p-n junctions are required, by doping such single crystals withextremely small and critical amounts of dopants.

These crystal growing processes produce such relatively small crystalsthat solar cells require the assembly of many single crystals toencompass the desired area of only a single solar cell panel. The amountof energy necessary to make a solar cell in this process, the limitationcaused by the size limitations of the silicon crystal, and the necessityto cut up and assemble such a crystalline material have all resulted inan impossible economic barrier to the large-scale use of the crystallinesemiconductor solar cells for energy conversion. Further, crystallinesilicon has an indirect optical edge which results in poor lightabsorption in the material. Because of the poor light absorption,crystalline solar cells have to be at least 50 microns thick to absorbthe incident sunlight. Even if the single crystal material is replacedby polycrystalline silicon with cheaper production processes, theindirect optical edge is still maintained; hence the material thicknessis not reduced. The polycrystalline material also involves the additionof grain boundaries and other problem defects.

In summary, crystalline silicon devices have fixed parameters which arenot variable as desired, require large amounts of material, are onlyproducible in relatively small areas and are expensive and timeconsuming to produce. Devices based upon amorphous silicon can eliminatethese crystalline silicon disadvantages. Amorphous silicon has anoptical absorption edge having properties similar to a direct gapsemiconductor and only a material thickness of one micron or less isnecessary to absorb the same amount of sunlight as the 50 micron thickcrystalline silicon. Further, amorphous silicon can be made faster,easier and in larger areas than can crystalline silicon.

Accordingly, a considerable effort has been made to develop processesfor readily depositing amorphous semiconductor alloys or films, each ofwhich can encompass relatively large areas, if desired, limited only bythe size of the deposition equipment, and which could be readily dopedto form p-type and n-type materials where p-n junction devices are to bemade therefrom equivalent to those produced by their crystallinecounterparts. For many years such work was substantially unproductive.Amorphous silicon or germanium (Group IV) films are normally four-foldcoordinated and were found to have microvoids and dangling bonds andother defects which produce a high density of localized states in theenergy gap thereof. The presence of a high density of localized statesin the energy gap of amorphous silicon semiconductor films results in alow degree of photoconductivity and short carrier lifetime, making suchfilms unsuitable for photoresponsive applications. Additionally, suchfilms cannot be successfully doped or otherwise modified to shift theFermi level close to the conduction or valence bands, making themunsuitable for making p-n junctions for solar cell and current controldevice applications.

In an attempt to minimize the aforementioned problems involved withamorphous silicon and germanium, W. E. Spear and P. G. Le Comber ofCarnegie Laboratory of Physics, University of Dundee, in Dundee,Scotland, did some work on "Substitutional Doping of Amorphous Silicon",as reported in a paper published in Solid State Communications, Vol. 17,pp. 1193-1196, 1975, toward the end of reducing the localized states inthe energy gap in amorphous silicon or germanium to make the sameapproximate more closely intrinsic crystalline silicon or germanium andof substitutionally doping the amorphous materials with suitable classicdopants, as in doping crystalline materials, to make them extrinsic andof p or n conduction types.

The reduction of the localized states was accomplished by glow dischargedeposition of amorphous silicon films wherein a gas of silane (SiH₄) waspassed through a reaction tube where the gas was decomposed by an r.f.glow discharge and deposited on a substrate at a substrate temperatureof about 500°-600° K. (227°-327° C.). The material so deposited on thesubstrate was an intrinsic amorphous material consisting of silicon andhydrogen. To produce a doped amorphous material, a gas of phosphine(PH₃) for n-type conduction or a gas of diborane (B₂ H₆) for p-typeconduction was premixed with the silane gas and passed through the glowdischarge reaction tube under the same operating conditions. The gaseousconcentration of the dopants used was between about 5×10⁻⁶ and 10⁻²parts per volume. The material so deposited included supposedlysubstitutional phosphorus or boron dopant and was shown to be extrinsicand of n or p conduction type.

While it was not known by these researchers, it is now known by the workof others that the hydrogen in the silane combines at an optimumtemperature with many of the dangling bonds of the silicon during theglow discharge deposition, to substantially reduce the density of thelocalized states in the energy gap toward the end of making theelectronic properties of the amorphous material approximate more nearlythose of the corresponding crystalline material.

The incorporation of hydrogen in the above method not only haslimitations based upon the fixed ratio of hydrogen to silicon in silane,but, more importantly, various Si:H bonding configurations introduce newantibonding states which can have deleterious consequences in thesematerials. Therefore, there are basic limitations in reducing thedensity of localized states in these materials which are particularlyharmful in terms of effective p as well as n doping. The resultingdensity of states of the silane deposited materials leads to a narrowdepletion width, which in turn limits the efficiencies of solar cellsand other devices whose operation depends on the drift of free carriers.The method of making these materials by the use of only silicon andhydrogen also results in a high density of surface states which affectsall the above parameters.

After the development of the glow discharge deposition of silicon fromsilane gas was carried out, work was done on the sputter depositing ofamorphous silicon films in the atmosphere of a mixture of argon(required by the sputtering deposition process) and molecular hydrogen,to determine the results of such molecular hydrogen on thecharacteristics of the deposited amorphous silicon film. This researchindicated that the hydrogen acted as an altering agent which bonded insuch a way as to reduce the localized states in the energy gap. However,the degree to which the localized states in the energy gap were reducedin the sputter deposition process was much less than that achieved bythe silane deposition process described above. The above-described p andn dopant gases also were introduced in the sputtering process to producep and n doped materials. These materials had a lower doping efficiencythan the materials produced in the glow discharge process. Neitherprocess produced efficient p-doped materials with sufficiently highacceptor concentrations for producing commercial p-n or p-i-n junctiondevices. The n-doping efficiency was below desirable acceptablecommercial levels and the p-doping was particularly undesirable since itreduced the width of the band gap and increased the number of localizedstates in the band gap.

Greatly improved amorphous silicon alloys having significantly reducedconcentrations of localized states in the energy gaps thereof andhigh-quality electronic properties have been prepared by glow dischargeas fully described in U.S. Pat. No. 4,226,898, Amorphous SemiconductorsEquivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and ArunMadan which issued Oct. 7, 1980, and by vapor deposition as fullydescribed in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky andMasatsugu Izu, which issued on Aug. 12, 1980, under the same title. Asdisclosed in these patents, which are incorporated herein by reference,fluorine is introduced into the amorphous silicon semiconductor tosubstantially reduce the density of localized states therein. Activatedfluorine especially readily diffuses into and bonds to the amorphoussilicon in the amorphous body to substantially decrease the density oflocalized defect states therein, because the small size of the fluorineatoms enables them to be readily introduced into the amorphous body. Thefluorine bonds to the dangling bonds of the silicon and forms what isbelieved to be a partially ionic stable bond with flexible bondingangles, which results in a more stable and more efficient compensationor alteration than is formed by hydrogen and other compensating oraltering agents. Fluorine is considered to be a more efficientcompensating or altering element than hydrogen when employed alone orwith hydrogen because of its exceedingly small size, high reactivity,specificity in chemical bonding, and highest electronegativity. Hence,fluorine is qualitatively different from other halogens and so isconsidered a super-halogen.

As an example, compensation may be achieved with fluorine alone or incombination with hydrogen with the addition of these element(s) in verysmall quantities (e.g., fractions of one atomic percent). However, theamounts of fluorine and hydrogen most desirably used are much greaterthan such small percentages so as to form a silicon-hydrogen-fluorinealloy. Such alloying amounts of fluorine and hydrogen may, for example,be in the range of 1 to 5 percent or greater. It is believed that thenew alloy so formed has a lower density of defect states in the energygap than that achieved by the mere neutralization of dangling bonds andsimilar defect states. Such larger amount of fluorine, in particular, isbelieved to participate substantially in a new structural configurationof an amorphous silicon-containing material and facilitates the additionof other alloying materials, such as germanium. Fluorine, in addition toits other characteristics mentioned herein, is believed to be anorganizer of local structure in the silicon-containing alloy throughinductive and ionic effects. It is believed that fluorine alsoinfluences the bonding of hydrogen by acting in a beneficial way todecrease the density of defect states which hydrogen contributes whileacting as a density of states reducing element. The ionic role thatfluorine plays in such an alloy is believed to be an important factor interms of the nearest neighbor relationships.

Amorphous semiconductor alloys made by the processes hereinabovedescribed have demonstrated photoresponsive characteristics ideallysuited for photovoltaic applications. These prior art processes,however, have suffered from relatively slow deposition rates and lowutilization of the reaction gas feed stock which are importantconsiderations from the standpoint of making photovoltaic andparticularly xerographic devices from these materials on a commercialbasis. In addition, these prior art processes, when practiced utilizinghigh RF power densities to enhance deposition rates result in theproduction of films with poor electrical properties, increased densitiesof defect states and in the production of powder in the reaction vesseldue to gas phase nucleation processes.

Many techniques of exciting conventional glow discharge plasmas havebeen investigated. These have included direct current (DC) andalternating current (AC) techniques. Various AC frequencies have beenutilized, such as audio, radio frequency (RF) and a microwave frequencyof 2.56 GHz. It is known that the optimum deposition power and pressureare defined by the minimum of the Paschen curve. The Paschen curvedefines the voltage (V) needed to sustain the glow discharge plasma ateach pressure (P) in a range of pressures, between electrodes separatedby a distance (D). In a typically sized, conventional RF glow dischargesystem, the minimum in the Paschen curve occurs at a few hundred mTorr.

The problem addressed by the present invention is how to achieve a highreaction gas conversion efficiency and a high deposition rate withoutsubstantially degrading the properties of the resulting alloys. It haspreviously been discovered that increasing the applied RF powerincreases the gas utilization efficiency and the deposition rate.However, simply increasing the RF power to achieve deposition ratesapproximately greater than 10 Å/sec. leads to the production ofamorphous semiconductor films of decreasing electronic quality and canresult in films which include polymeric material and/or the productionof powder. The increased deposition rate with increased RF power is aresult of an increase in the concentration of excited species resultingprincipally from collisions between electrons and feedstock molecules.However, the collision rate between excited species and more importantlybetween excited species and feedstock molecules is also increased. Thisresults in the formation of polymer chains. These chains are eitherincorporated in the growing amorphous semiconductor film degrading itselectronic quality or condensed in the gas phase to produce powderparticles. To reduce the number of undesirable collisions one can reducethe operating pressure, but this moves the deposition process off theminimum of the Paschen curve and substantially higher RF power isrequired to achieve the same degree of plasma excitation. The physicalreason for this phenomenon is that, as pressure is reduced, manyelectrons that would have been able to collisionally excite feedstockmolecules at higher gas pressures now impinge on the substrate or systemwalls without suffering collisions.

Attempts have also been made to increase the gas utilization efficiencyin RF glow discharge plasmas by high power deposition of a dilutemixture of silane (SiH₄) in an inert carrier gas such as argon. However,this is known to result in undesirable film growth conditions givingrise to columnar morphology as reported by Knights, Journal ofNon-Crystalline Solids, Vol. 35 and 36, p. 159 (1980).

The one group which has reported glow discharge amorphous silicon alloydeposition utilizing microwave energy at 2.54 GHz treated the microwaveenergy as just another source of plasma excitation by performing thedeposition in a plasma operating at pressures typical of conventional RFor DC glow discharge processes. C. Mailhiot et al. in the Journal ofNon-Crystalline Solids, Vol. 35 and 36, p. 207-212 (1980) describe filmsdeposited at 0.17 Torr to 0.30 Torr at deposition rates of between 23and 34 Å/sec. They report that their films, which are of poor electricalquality, show clear indication of non-homogeneous structure. Thus,Mailhiot et al. failed to discover the present invention which is basedon the recognition that for a given deposition system the minimum in thePaschen curve shifts to lower pressure values with increasing frequency.Therefore, the use of high frequency microwave energy in a glowdischarge deposition system allows one to operate at much lower pressureand consequently to achieve a higher concentration of excited speciesand thus higher deposition rate and gas utilization efficiency withoutproduction of powder or inclusion of polymeric species in the amorphoussemiconductor film. The shift in the minimum of the Paschen curve isbelieved to occur because, for a given gas pressure at the higherexcitation frequency, the rapid reversals of the applied electric fieldallow the electrons in the plasma to collide with more feedstockmolecules in the plasma excitation region before they encounter thewalls of the system. Thus, the present invention provides both asubstantially increased deposition rate of 100 Å/sec. or above and afeedstock conversion efficiency approaching 100% while still allowingthe production of high electrical quality amorphous semiconductor films.This contrasts both with the conventional RF (eg., 13.56 MHz, 0.2 to 0.5Torr) glow discharge deposition process which produces high qualityfilms at deposition rates of approximately 10 Å/sec. and feedstockutilization of approximately 10% and with the Mailhiot microwave process(2.54 GHz, 0.2 Torr to 0.3 Torr) which produced poor quality films at 20to 30 Å/sec. deposition rates.

Applicants herein have discovered a new and improved process for makingamorphous semiconductor alloys and devices. The inventive process hereinprovides substantially increased deposition rates and reaction gas feedstock utilization. Further, the process of the present invention resultsin depositions from plasmas operated at lower pressure and consequentlycapable of powderless depositions of a semiconductor film with onlysmall amounts of incorporated polymeric material. Still further, theprocess of the present invention results in the formation of reactivespecies not previously obtainable in sufficiently large concentrationswith prior art processes. As a result, new amorphous semiconductoralloys can be produced having substantially different materialproperties than previously obtainable. All of the above results, byvirtue of the present invention, in amorphous semiconductor alloys anddevices made therefrom having improved photoresponsive characteristicswhile being made at substantially increased rates.

As disclosed in the aforementioned referenced U.S. Pat. No. 4,217,374,new and improved amorphous semiconductor alloys and devices can be madewhich are stable and composed of chemical configurations which aredetermined by basic bonding considerations. One of these considerationsis that the material is as tetrahedrally bonded as possible to permitminimal distortion of the material without long-range order. Fluorine,for example, when incorporated into these alloy materials, performs thefunction of promoting tetrahedral bonding configurations. Amorphoussemiconductor materials having such tetrahedral structures exhibit lowdensities of dangling bonds making the materials suitable forphotovoltaic applications.

In accordance with one preferred embodiment, atomic fluorine and/orhydrogen are generated separately as free radicals and reacted withsemiconductor free radicals generated within a plasma sustained bymicrowave energy. As a result, all of the advantages of separate freeradical formation are obtained along with all of the advantages ofmicrowave plasma deposition.

In making conventional xerographic devices, materials such as variousamorphous chalcogenide alloys including Se₉₂ Te₈ ; As₂ Se₃ ; or organicalloys such as TNF-PVK have been utilized. These materials are soft,easily damaged in use, and toxic. Amorphous silicon alloys are much moredesirable for this application since they are hard, non-toxic, andcapable of being formulated with a wide range of spectral responsecharacteristics. Low deposition rates and low gas utilizationefficiencies have, however, hindered their commercial application. Onepreferred embodiment of the invention makes possible the formation ofelectrophotographic members by the high gas utilization efficiency andthe high deposition rate achieved by the present invention.

In making a commercial photovoltaic device, it is necessary to provideenvironmental encapsulation of the devices to prevent undesirablechemical reactions within the device materials due to exposure tochemical species contained in the environment. For example, oxidation ofcontact materials must be prevented. Customarily, relatively heavy andthick materials such as glass plates or various organic polymer layersor plastic materials have been proposed to provide such protection. Inaccordance with a further embodiment of the present invention, suchprotection is provided by microwave glow discharge deposition ofappropriately hard, chemically inert wide bandgap semiconductormaterial, deposited at high deposition rates. The microwave depositedmaterial not only provides the required encapsulation, but isadditionally light in weight and can be easily incorporated in a mannercompatible with the formation of the photovoltaic materials of thedevices.

SUMMARY OF THE INVENTION

The present invention provides a process for making amorphoussemiconductor alloy films and devices at substantially higher rates thanpossible in the prior art. Even though deposition occurs at the higherrates, the alloys exhibit high-quality electronic properties suitablefor many applications including photovoltaic and electrophotographicapplications.

In accordance with the invention, the process includes the steps ofproviding a source of microwave energy, coupling the microwave energyinto a substantially enclosed reaction vessel containing the substrateonto which the amorphous semiconductor film is to be deposited, andintroducing into the vessel at least one reaction gas and evacuating thevessel to a low enough deposition pressure to deposit the film at highdeposition rates with high reaction gas conversion efficiencies withoutany significant powder or polymeric inclusions. The microwave energy andthe reaction gases form a glow discharge plasma within the vessel todeposit an amorphous semiconductor film from the reaction gases onto thesubstrate. The reaction gases can include silane (SiH₄), silicontetrafluoride (SiF₄), silane and silicon tetrafluoride, silane andgermane (GeH₄), or silicon tetrafluoride and germane. The reaction gasescan also include germane or germanium tetrafluoride (GeF₄). To all ofthe foregoing, hydrogen (H₂) can also be added. Dopants, either p-typeor n-type can also be added to the reaction gases to form p-type orn-type alloy films, respectively. Also, band gap increasing elementssuch as carbon or nitrogen can be added in the form of, for example,methane or ammonia gas to widen the band gap of the alloys.

Independent control over all of the deposition parameters can beobtained by separately generating the free radical species prior tocombination in the microwave plasma. For example, atomic fluorine and/orhydrogen can be separately generated and fed into the plasma wherein thesemiconductor free radicals are generated. The foregoing thereafterreact in the plasma and are deposited onto the substrate to form new andimproved amorphous semiconductor alloys. The semiconductor free radicalscan be generated from any of the semiconductor-containing compoundspreviously mentioned.

Also, encapsulation of the photovoltaic devices is obtained bydeposition of a relatively thin layer of transparent insulatingmaterials over the devices. For example, the transparent materials canbe silicon nitride (Si₃ N₄) or silicon dioxide (SiO₂) formed, forexample, by the microwave glow discharge of silane and ammonia ornitrogen and silane and oxygen respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view, partly broken away, of a microwave plasmadeposition system for depositing amorphous semiconductor alloy films inaccordance with the process of the invention;

FIG. 2 is a fragmentary sectional view of an embodiment of a Schottkybarrier solar cell to illustrate one application of the amorphoussemiconductor photoreceptive alloys made by the process of theinvention.

FIG. 3 is a fragmentary sectional view of a p-n junction solar celldevice which includes a doped amorphous semiconductor alloy made by theprocess of the invention;

FIG. 4 is a fragmentary sectional view of a photodetection device whichincludes an amorphous semiconductor alloy made by the process of theinvention;

FIG. 5 is a fragmentary sectional view of a xerographic drum includingan amorphous semiconductor alloy made by the process of the invention;

FIG. 6 is a fragmentary sectional view of a p-i-n junction solar celldevice;

FIG. 7 is a fragmentary sectional view of an n-i-p junction solar celldevice;

FIG. 8 is a partial top plan view of an alternative gas feed arrangementfor the apparatus of FIG. 1 in accordance with a further embodiment ofthe present invention;

FIG. 9 is a partial top plan view of a free radical distribution systemfor the apparatus of FIG. 1 in accordance with another embodiment of thepresent invention;

FIG. 10 is a partial perspective view of another microwave plasmadeposition system in accordance with a still further embodiment of thepresent invention;

FIG. 11 is a modified Paschen curve for conventional RF depositionconditions; and

FIG. 12 is a modified Paschen curve for the microwave depositionconditions of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now more particularly to FIG. 1, a microwave depositionapparatus suitable for practicing the process of the present inventionis generally designated 10. The apparatus 10 comprises a transparenttubular chamber or vessel 12 containing a substrate 14 upon which theamorphous semiconductor alloy films are to be deposited. The substrateis heated by a radiant heater 16 and the exterior of the chamber isirradiated by a microwave energy source 17. Reaction gases passing frominlets 46 to an outlet 20 at opposite ends of the chamber 12 receivemicrowave energy from the source 17 in the area of the substrate. Thecombination of the reaction gases and the microwave energy from source17 causes the formation of a plasma in the area of the substrate,resulting in the deposition of a film 22. In accordance with the presentinvention, the reaction gases include at least onesemiconductor-containing compound to form the plasma. The temperature ofthe substrate can be between room temperature and about 400° C. and thefrequency of the microwave energy can be 2.45 GHz and above andpreferably 2.45 GHz. As mentioned, the combined microwave energy and thereaction gases form the plasma to permit the deposition process toproceed.

Referring now to FIG. 1 in greater detail, the tubular chamber 12comprises a central quartz portion 24 and end portions 26 at oppositeends thereof. The end portions 26 are closed by a pair of end fittings30 and 32 to complete the chamber. Each of the end fittings includes asleeve portion 34 extending from a closed end 36 to an open end portion.The open end portion is threaded to receive a collar 40 having aninwardly extending annular flange 42 at one end thereof. An o-ring (notshown) is confined in a space between the flange 42 and the end portionfor compression thereof against the quartz portion 26. An airtight sealis provided in this way between the end fittings 30 and 32 and thetubular chamber 12.

The end fittings 30 and 32 are preferably made of stainless steel orother suitable noncorrosive metal, with the closed ends 36 being weldedor otherwise permanently joined to the sleeve portions 34. The closedend 36 of the end fitting 32 is provided with gas inlets 46 throughwhich the deposition gases are introduced into the vessel 12. An inertgas such as argon can be introduced through one inlet 46 to assist insustaining the plasma.

The gas inlets 46 are preferably connected to a conventional gas rack(not shown) for establishing regulated flows of reaction gases therein.The outlet 20 is provided at the closed end 36 to the end fitting 30 forconnection to selectable first and second pumps. The first pump providesfor initial evacuation of the chamber. The second pump provideswithdrawal of unused reaction gases during operation and maintenance ofthe proper deposition pressure of 0.1 Torr or less.

The microwave energy source 17 preferably comprises a microwave energygenerator 18 coupled to an antenna 19. The antenna 19 is housed within areflective housing 21 for concentration of the microwave energy into thechamber 12. The antenna as illustrated is a vertical antenna beingpreferably one-quarter wavelength long. The tip of the antenna justtouches the outer surface of the vessel 12 to maximize transmission ofits output to the reaction gases.

The radiant heater 16 preferably comprises a conventional resistiveheater. Heat is transmitted to the substrate 14 and the chamber 12 byradiation, without significant direct heating of the reaction gases.Alternatively, a resistive heating arrangement (not shown) may beprovided within the chamber 12 for heating the substrate 14. In thatcase, power lines for the heating element would be passed through theclosed end 36 of one of the end fittings.

In operation, the system 10 is first pumped down to below a desireddeposition pressure, such as 10⁻⁵ Torr. The reaction gas mixture orgases such as silicon tetrafluoride (SiF₄), silane (SiH₄), silicontetrafluoride and silane, silane and germane (GeH₄), or silicontetrafluoride and germane are fed into the inlet chamber 24 throughseparate inlet conduits 46 and chamber 12 is brought up to the desiredoperating pressure. To the foregoing reaction gases, hydrogen (H₂) canalso be added. Other reaction gases which can be utilized are germane,germanium tetrafluoride (GeF₄), germanium tetrafluoride and silicontetrafluoride. Hydrogen (H₂) can also be added to these gases.

The microwave energy from the antenna 19 is directed by the reflectivehousing 21 into the vessel 12 to form a plasma in the area of thesubstrate. As a result, an amorphous semiconductor alloy film 22 isdeposited onto the substrate 14. The heater 16 maintains the substrateat a temperature between about 20° C. and 400° C., and preferablybetween 250° C. and 325° C. The output power of the microwave energygenerator 18 is adjusted between about 20 and 115 watts depending on thevolume of the gas contained in the plasma and the composition of thegas. These power outputs preferably correlate to about 0.1 to 1.2 wattsper cubic centimeter in power density. The flow rate of the reactiongases can be between 1 to 50 SCCM. The vessel is evacuated to anoperating pressure of 0.001 to 0.1 Torr and preferably about 0.05 Torror less. With the foregoing system parameters, deposition rates of 50 Åup to 250 Å per second or higher as controlled by the gas flow rate canbe obtained. The reaction gas is converted to deposition species at aconversion efficiency of essentially 100%. Even at these high depositionrates, the deposited amorphous semiconductor films do not have anysignificant powder or polymeric inclusions and exhibit high-qualityphotoresponsive characteristics suitable for photovoltaic and otherapplications.

For making photovoltaic or other doped devices by the process of theinvention, dopants can be introduced into the vessel 12 for making thedeposited film either p-type or n-type. For example, to make a p-typefilm, diborane gas (B₂ H₆) can be introduced through one of the inlets46. For making n-type films, phosphine gas (PH₃) can be introduced intoone of the inlets 46. If it is desired to increase the band gap of amaterial, band gap increasing elements such as carbon or nitrogen can beincorporated into the films by introducing methane (CH₄) or ammonia(NH₃) into one of the inlets 46 during deposition. The reaction gasescan be sequentially introduced to result in the formation of any desireddevice configuration.

To assist in the maintenance of the plasma, a plasma sustaining gas canbe introduced into one of the inlets 46. Argon gas can be utilized forthis purpose. The deposition of amorphous semiconductor alloy films bythe foregoing inventive process has demonstrated many advantages overprior deposition processes. Firstly, the microwave energy provides ahigher density of free radicals than previously possible. This resultsin higher deposition rates, nearly total utilization of the feed stockreaction gases and enhanced reactivity of plasma species leading toincorporation in the growing film of elements which previously could notbe so incorporated. This results in new materials having uniquecompositional and structural properties. In summary, the inventionprovides a process for making amorphous semiconductor films and devicesat higher deposition rates and reaction gas conversion efficiencieswhich have improved properties and which afford wide variations in thematerial compositions.

Various applications of the improved amorphous alloys produced by theunique processes of the invention are illustrated in FIGS. 2 through 7.FIG. 2 shows a Schottky barrier solar cell 142 in fragmentary crosssection. The solar cell 142 includes a substrate or electrode 144 of amaterial having good electrical conductivity properties, and the abilityof making an ohmic contact with an amorphous alloy 146 compensated oraltered to provide a low density of localized states in the energy gap.The substrate 144 may comprise a low work function metal, such asaluminum, tantalum, stainless steel or other material matching with theamorphous alloy 146 deposited thereon which preferably includes silicon,compensated or altered in the manner of the alloys previously describedso that it has a low density of localized states in the energy gap ofpreferably no more than 10¹⁶ per cubic centimeter per eV. It is mostpreferred that the alloy have a region 148 next to the electrode 144,which region forms an n-plus conductivity, heavily doped, low-resistanceinterface between the electrode and an undoped, relatively high, darkresistance region 150 which is an intrinsic, but low n-conductivityregion.

The upper surface of the amorphous alloy 146 as viewed in FIG. 2, joinsa metallic region 152, an interface between this metallic region and theamorphous alloy 146 forming a Schottky barrier 154. The metallic region152 is transparent or semi-transparent to solar radiation, has goodelectrical conductivity and is of a high work function (for example, 4.5eV or greater, produced, for example, by gold, platinum, palladium,etc.) relative to that of the amorphous alloy 146. The metallic region152 may be a single layer of a metal or it may be a multilayer. Theamorphous alloy 146 may have a thickness of about 0.5 to 1 micron andthe metallic region 152 may have a thickness of about 100 Å in order tobe semi-transparent to solar radiation.

On the surface of the metallic region 152 is deposited a grid electrode156 made of a metal having good electrical conductivity. The grid maycomprise orthogonally related lines on conductive material occupyingonly a minor portion of the area of the metallic region, the rest ofwhich is to be exposed to solar energy. For example, the grid 156 mayoccupy only about from 5 to 10% of the entire area of the metallicregion 152. The grid electrode 156 uniformly collects current from themetallic region 152 to assure a good low series resistance for thedevice.

An anti-reflection layer 158 may be applied over the grid electrode 156and the areas of the metallic region 152 between the grid electrodeareas. The anti-reflection layer 158 has a solar radiation incidentsurface 160 upon which impinges the solar radiation. For example, theanti-reflection layer 158 may have a thickness on the order of magnitudeof the wavelength of the maximum energy point of the solar radiationspectrum, divided by four times the index of refraction of theanti-reflection layer 158. If the metallic region 152 is platinum of 100Å in thickness, a suitable anti-reflection layer 158 would be zirconiumoxide of about 500 Å in thickness with an index of refraction of 2.1.

The Schottky barrier 154 formed at the interface between the regions 150and 152 enables the photons from the solar radiation to produce currentcarriers in the alloy 146, which are collected as current by the gridelectrode 156. An oxide layer (not shown) can be added between thelayers 150 and 152 to produce an MIS (metal insulator semiconductor)solar cell.

In addition to the Schottky barrier or MIS solar cell shown in FIG. 2,there are solar cell constructions which utilize p-n junctions in thebody of the amorphous alloy forming a part thereof formed in accordancewith successive deposition, compensating or altering and doping stepslike that previously described. These other forms of solar cells aregenerically illustrated in FIG. 3 as well as in FIGS. 6 and 7.

These constructions, such as a solar cell 162 generally include atransparent electrode 164 through which the solar radiation energypenetrates into the body of the solar cell involved. Between thistransparent electrode and an opposite electrode 166 is a depositedamorphous alloy 168, preferably including silicon, initially compensatedin the manner previously described. In this amorphous alloy 168 are atleast two adjacent regions 170 and 172 where the amorphous alloy hasrespectively oppositely doped regions, region 170 being shown as ann-conductivity region and region 172 being shown as a p-conductivityregion. The doping of the regions 170 and 172 is only sufficient to movethe Fermi levels to the valence and conduction bands involved so thatthe dark conductivity remains at a low value achieved by the bandadjusting and compensating or altering method of the invention. Thealloy 168 has high conductivity, highly doped ohmic contact interfaceregions 174 and 176 of the same conductivity type as the adjacent regionof the alloy 168. The alloy regions 174 and 176 contact electrodes 164and 166, respectively.

Referring now to FIG. 4, there is illustrated another application of anamorphous alloy utilized in a photodetector device 178 whose resistancevaries with the amount of light impinging thereon. An amorphous alloy180 thereof is deposited in accordance with the invention, has no p-njunctions as in the embodiment shown in FIG. 3 and is located between atransparent electrode 182 and a substrate electrode 184. In aphotodetector device it is desirable to have a minimum darkconductivity, so the amorphous alloy 180 has an undoped, but compensatedor altered region 186 and heavily doped regions 188 and 190 of the sameconductivity type forming a low-resistance ohmic contact with theelectrodes 182 and 184, which may form a substrate for the alloy 180.

Referring to FIG. 5, an electrostatic image-producing device 192 (like axerography drum) is illustrated. The device 192 has a low darkconductivity, selective wavelength threshold, undoped or slightlyp-doped amorphous, oxygen stabilized alloy 194, deposited on a suitablesubstrate 196 such as a drum.

As used herein, the terms compensating agents or materials and alteringagents, elements or materials mean materials which are incorporated inthe amorphous alloy for altering or changing the structure thereof, suchas, activated fluorine (and hydrogen) incorporated in the amorphous,alloy-containing silicon to form an amorphous silicon/fluorine/hydrogencomposition alloy, having a low density of localized states in theenergy gap. The activated fluorine (and hydrogen) is bonded to thesilicon in the alloy and reduces the density of localized statestherein; and due to the small size of the fluorine and hydrogen atoms,they are both readily introduced into the amorphous alloy withoutsubstantial dislocation of the silicon atoms and their relationships inthe amorphous alloy.

Referring now to FIG. 6, a p-i-n solar cell 198 is illustrated having asubstrate 200 which may be glass or a flexible web formed from stainlesssteel or aluminum. The substrate 200 is of a width and length as desiredand preferably at least 3 mils thick. The substrate has an insulatinglayer 202 deposited thereon by a conventional process such as chemicaldeposition, vapor deposition or anodizing in the case of an aluminumsubstrate. The layer 202 for instance, about 5 microns thick can be madeof a metal oxide and can be eliminated if desired. For an aluminumsubstrate, it preferably is aluminum oxide (Al₂ O₃) and for a stainlesssteel substrate it may be silicon dioxide (SiO₂) or other suitableglass.

An electrode 204 is deposited in one or more layers upon the layer 202to form a base electrode for the cell 198. The substrate 200 can alsoform the bottom electrode without the layers 202 and 204. The electrode204 layer or layers is deposited by vapor deposition, which is arelatively fast deposition process. The electrode layers preferably arereflective metal electrodes of molybdenum, aluminum, chrome or stainlesssteel for a solar cell or a photovoltaic device. The reflectiveelectrode is preferable since, in a solar cell, non-absorbed light whichpasses through the semiconductor alloy is reflected from the electrodelayers 204 where it again passes through the semiconductor alloy whichthen absorbs more of the light energy to increase the device efficiency.

The substrate 200 is then placed in the deposition environment. Thespecific examples shown in FIGS. 6 and 7 are only illustrative of somep-i-n junction devices which can be manufactured utilizing the improvedprocess of the invention. For example, tandem cells can also be made bythe process of the present invention. Each of the devices illustrated inFIGS. 6 and 7 has an alloy body having an overall thickness of betweenabout 3,000 and 30,000 Å. This thickness ensures that there are nopinholes or other physical defects in the structure and that there ismaximum light absorption efficiency. A thicker material may absorb morelight, but at some thickness will not generate more current since thegreater thickness allows more recombination of the light-generatedelectron-hole pairs. (It should be understood that the thicknesses ofthe various layers shown in FIGS. 2 through 7 are not drawn to scale.)

Referring first to forming the n-i-p device 198, the device is formed byfirst depositing a heavily doped n⁺ alloy layer 206 on the electrode204. Once the n⁺ layer 206 is deposited, an intrinsic (i) alloy layer208 is deposited thereon. The intrinsic layer 208 is followed by ahighly doped conductive p⁺ alloy layer 210 deposited as the finalsemiconductor layer. The amorphous alloy layers 206, 208 and 210 formthe active layers of the n-i-p device 198.

While each of the devices illustrated in FIGS. 6 and 7 may have otherutilities, they will be now described as photovoltaic devices. Utilizedas a photovoltaic device, the selected outer, p⁺ layer 210 is a lowlight absorption, high conductivity alloy layer. The intrinsic alloylayer 208 preferably has an adjusted wavelength threshold for a solarphotoresponse, high light absorption, low dark conductivity and highphotoconductivity. The bottom alloy layer 204 is a low light absorption,high conductivity n⁺ layer. The overall device thickness between theinner surface of the electrode layer 206 and the top surface of the p⁺layer 210 is, as stated previously, on the order of at least about 3,000Å. The thickness of the n⁺ doped layer 206 is preferably in the range ofabout 50 to 500 Å. The thickness of the amorphous, intrinsic alloy 208is preferably between about 3,000 to 30,000 Å. The thickness of the topp⁺ contact layer 210 also is preferably between about 50 to 500 Å. Dueto the shorter diffusion length of the holes, the p⁺ layer generallywill be as thin as possible on the order of 50 to 150 Å. Further, theouter layer (here p⁺ layer 210) whether n⁺ or p⁺ will be kept as thin aspossible to avoid absorption of light in the contact layer.

A second type of p-i-n junction device 212 is illustrated in FIG. 7. Inthis device a first p⁺ layer 214 is deposited on the electrode layer204' followed by an intrinsic amorphous alloy layer 216, an n amorphousalloy layer 218 and an outer n⁺ amorphous alloy layer 220. Further,although the intrinsic alloy layer 208 or 216 (in FIGS. 6 and 7) is anamorphous alloy, the other layers are not so restricted and could, forinstance, be polycrystalline, such as layer 214. (The inverse of theFIGS. 6 and 7 structure not illustrated, also can be utilized.)

Following the deposition of the various semiconductor alloy layers inthe desired order for the devices 198 and 212, a further deposition stepis performed, preferably in a separate deposition environment.Desirably, a vapor deposition environment is utilized since it is a fastdeposition process. In this step, a TCO layer 222 (transparentconductive oxide) is added which, for example, may be indium tin oxide(ITO), cadmium stannate (Cd₂ SnO₄), or doped tin oxide (SnO₂). The TCOlayer will be added following the post compensation of fluorine (andhydrogen) if the films were not deposited with one or more of thedesired compensating or altering elements therein. Also, the othercompensating or altering elements, above described, can be added by postcompensation.

An electrode grid 224 can be added to either of devices 198 or 212 ifdesired. For a device having a sufficiently small area, the TCO layer222 is generally sufficiently conductive such that the grid 224 is notnecessary for good device efficiency. If the device is of a sufficientlylarge area or if the conductivity of the TCO layer 222 is insufficient,the grid 224 can be placed on the layer 222 to shorten the carrier pathand increase the conduction efficiency of the devices.

Lastly, a transparent encapsulant 225 is deposited over the grid 224.This encapsulant can comprise, for example, silicon nitride (Si₃ N₄) orsilicon dioxide (SiO₂) formed from the microwave deposition of silaneand nitrogen or ammonia or silane and oxygen respectively. Theseencapsulants can be called insulators or wide band gap semiconductors.The layer 225 of transparent material can have a thickness of about oneto fifty microns.

If the layer 225 is formed from silicon nitride or silicon dioxide, thedeposition temperature can be room temperature to 300° C. and thedeposition pressure preferably can be between 0.001 and 0.04 Torr. Thereaction gas flow range can be about 2-10 SCCM and the power density canbe about 0.1 to 1.2 watts per cubic centimeter. For silicon nitride, thereaction gas mixture can be from 0.1 to 50% SiH₄ in N₂.

Each of the device semiconductor alloy layers can be deposited upon thesubstrate by the apparatus illustrated in FIG. 1. The vessel 12initially is evacuated to purge or eliminate impurities in theatmosphere from the deposition system. The alloy material preferably isthen fed into the deposition chamber in a compound gaseous form, mostadvantageously as semiconductor containing compounds for intrinsicmaterials. The reactive gas can contain band gap narrowing elements suchas germanium to form an intrinsic, amorphous semiconductor alloy havinga narrowed band gap. The microwave generator is energized and the plasmais obtained from the gas mixture. The band gap narrowing and increasingelements can be called band gap adjusting elements.

The semiconductor material is deposited from the plasma onto thesubstrate which can be heated to the desired deposition temperature foreach alloy layer. For example, the substrate temperature can be 275° C.for amorphous silicon and germanium alloys and 200° C. for amorphousgermanium alloys deposited from GeF₄ or GeH₄. The doped layers of thedevices are deposited at various temperatures of for example 250° C. to300° C. depending upon the form of the material used. The upperlimitation on the substrate temperature in part is due to the type ofmetal substrate utilized. For an initially hydrogen compensatedamorphous alloy to be produced, such as to form the intrinsic layer inn-i-p or p-i-n devices, the substrate temperature should be less thanabout 400° C. and preferably about 275° C.

The doping concentrations are varied to produce the desired p, p⁺, n orn⁺ type conductivity as the alloy layers are deposited for each device.For n or p doped layers, the material is doped with 5 to 100 ppm ofdopant material as it is deposited. For n⁺ or p⁺ doped layers thematerial is doped with 100 ppm to over 1 percent of dopant material asit is deposited.

Referring now to FIG. 8, there is illustrated an alternative gas feedsystem for the apparatus of FIG. 1. The gas feed system includes a gasdistribution manifold 230 within the enclosed chamber 24. The manifold230 has an extension 232 which extends through the chamber end cap (notshown) for receiving the various gas mixtures to be utilized in themicrowave plasma. The gas mixtures can be any of the gas mixturespreviously identified. As can be seen in FIG. 8, the manifold loopsaround the substrate 14 and includes a plurality of outlets alongsubstantially parallel portions 234 and 236. This allows the reactiongases indicated by arrows 238 to be evenly distributed over thesubstrates to result in a more uniform plasma. As a result, theamorphous semiconductor alloy film deposited onto the substrate 14 willhave uniform electrical and optical properties across the substrate 14.Such an arrangement is advantageous when using gases such as, forexample, silicon tetrafluoride and germane, or silicon tetrafluoride andgermanium tetrafluoride wherein the silicon compounds and germaniumcompounds have different disassociation energies and consequently wouldotherwise result in a deposited film which exhibits a compositionalnon-uniformity in the direction of feed gas flow across the substrate.

Referring now to FIG. 9, there is illustrated a system of feeding intothe plasma atomic fluorine and/or hydrogen which have been separatelygenerated. The system includes a pair of conduits 240 and 242 whichextend into the chamber 24 on opposite sides of the substrate 14. Theconduits 240 and 242 are substantially equally spaced from the substrateand include outlets in the vicinity of the substrate for evenlydistributing atomic fluorine and/or hydrogen (indicated by arrows 244and 246) into the plasma over the substrate 14. The atomic fluorineand/or hydrogen can then react with the semiconductor free radicalswithin the plasma disassociated from the semiconductor-containing gases248 fed into the chamber 24 through inlets 46 (not shown). The atomicfluorine and/or hydrogen and the semiconductor free radicals react toform a film on the substrate. As a result, the system of FIG. 9 providesseparate control over the free radicals within the plasma to enableselective incorporation of desired species into the plasma from whichthe film is deposited.

Other free radicals can of course be introduced by adding additionalconduits. For example, free radicals of boron can be introduced toprovide substitutional doping within the deposited film to form animproved p-type alloy. Such an alloy is particularly useful in makingphotovoltaic devices.

Referring now to FIG. 10, there is illustrated another microwavedeposition system 249 in accordance with a further embodiment of theinvention. In this system a free radical generator 250, including a"Woods Horn" 254 known in the art, is used to feed selected freeradicals 252 into the chamber 24. Of course, additional generators 250can be provided. A microwave source as in FIG. 1 can be providedincluding a microwave generator 18, an antenna 19, and a reflectivehousing 21. The generator 18 can provide the free radical generator 250with microwave energy, or the free radical generator 250 can include itsown source of microwave energy.

The free radicals 252 react with the reactive species formed within theplasma from the reaction gases 256 to form a film on the substrate 14.Hence, as in the previous embodiment, selected free radicals can beintroduced into the plasma at will to form new and improved amorphoussemiconductor alloys.

The atomic fluorine and/or hydrogen within the plasma provides amorphoussemiconductor alloys having improved structural and chemical properties.Infra-red spectroscopy shows a significant silicon-fluorine peak in thealloys indicating that the fluorine is bonding to the silicon in apreferred manner providing material stability and reduced density ofstates. This is of particular importance in the fabrication ofphotovoltaic devices.

FIG. 11 illustrates a curve 260 which is produced utilizing conventionalRF deposition techniques of SiH₄, which is a modified Paschen curve. Asabove mentioned, a Paschen curve is the voltage needed to sustain aplasma at each pressure. The modified curve is related to the powerneeded to sustain a plasma at each pressure. The normal operating rangeis dictated by the minimum of the curve, which for the curve shown isabout 0.2 to 0.25 Torr.

FIG. 12 illustrates a modified Paschen curve 262 for the microwavedeposition process of the invention, again utilizing SiH₄ as thereaction gas. The curve 262 shows the clear shift in the curve to lowerpressure for the microwave deposition process of the invention, whichresults in an operating range of about 0.075 to 0.1 Torr or below forthe invention. The curve shifts for other reaction gases and depositionsystem dimensions and lower pressures are preferred.

FIGS. 11 and 12 clearly show that Mailhiot failed to discover thepresent invention. The Mailhiot et al. published results indicatedpressure ranges of 0.17 to 0.30 Torr to achieve deposition rates of 23to 34 Å. The density of states of this material was not published, butit was stated that the results were "not compatible with a single,homogeneous a-Si:H film . . . " That statement and the high activationenergy of 0.1 eV would lead one to conclude that the film had more thanone phase which would indicate polymer and/or powder inclusions.

A second article published two years later, by the same group, M. Akliket al., Journal of Applied Physics, Vol. 53(1) p. 439-441 describesfilms which do have good RF qualities, with a density of states of 10¹⁷cm⁻³ eV⁻¹. This film was, however, deposited essentially under RFconditions, with a deposition rate of 10 Å per second at a pressure of0.27 Torr. Clearly, the microwave source is merely being utilized asanother type of deposition source without any appreciation of the highdeposition rates and high gas conversion efficiencies (which are notmentioned) provided by the present invention.

Amorphous semiconductor alloy films can be deposited by the presentinvention in the following ranges of operating parameters.

Pressure: 0.001 to 0.1 Torr

Deposition Rate: 50 Å/sec. to 250 Å and above

Substrate Temperature: 250° C. to 325° C.

Power Density: 0.1 w/cm³ to 1.2 w/cm³

Reaction Gas Conversion Efficiency: Essentially or substantially 100%.

Examples of some specific films deposited by the process of theinvention are:

an a-Si:H alloy film deposited from SiH₄ at 120 Å/sec., at 0.05 Torr, at300° C.;

an a-Si:H:F alloy film deposited from a mixture of 2 parts SiF₄ to 9parts SiH₄ at 125 Å/sec., at 0.065 Torr, at 300° C.;

an Si₃ N₄ alloy film deposited from equal parts of SiH₄ and N₂ at 133Å/sec., at 0.044 Torr, at 300° C.;

an SiO₂ alloy film deposited from five parts of an equal mixture of O₂in SiF₄ and 1 part H₂ at 55 Å/sec., at 0.04 Torr, at 300° C. The a-Si:Hand a-Si:H:F alloy films made by the present invention have activationenergies of 0.7 to 0.8 eV, σ_(L) (under AM 1) of about 3×10⁻⁵ or betterand σ_(D) of about 5×10⁻¹⁰ or better which indicates alloys of goodelectronic quality in contrast to those materials of Mailhiot. Tofurther emphasize the effects of the pressure difference, two alloyfilms were deposited in the same system with only the pressure beingchanged. The alloy films were both deposited at about the same rate of150 Å/sec. from a mixture of 30 parts SiH₄ to 7.2 parts of SiF₄ at 300°C. The first alloy film was deposited at 0.023 Torr and had a σ_(L) of4×10⁻⁶ and a σ_(D) of 4×10⁻¹¹. The second alloy film was deposited at0.12 Torr and had a σ_(L) of 5×10⁻⁷ and a σ_(D) of 1.3×10⁻⁹, caused bythe increase in defect density.

Modifications and variations of the present invention are possible inlight of the above teachings. As previously mentioned, the alloy layersother than the intrinsic alloy layer can be other than amorphous layers,such as polycrystalline layers. (By the term "amorphous" is meant analloy or material which has long-range disorder, although it may haveshort or intermediate order or even contain at times some crystallineinclusions.) It is therefore, to be understood that within the scope ofthe appended claims the invention may be practiced otherwise than asspecifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A process for depositing amorphous semiconductoralloy films onto a substrate, comprising:providing a source of microwaveenergy; coupling said microwave energy into a substantially enclosedreaction vessel containing the substrate; introducing into said vesselat least one reaction gas to form a glow discharge plasma within saidvessel to form reaction gas species from said reaction gas; andevacuating said reaction vessel to a deposition pressure of 0.1 torr orless so as to provide for the deposition of an amorphous semiconductoralloy film from said reaction gas species onto said substrate at highdeposition rates with high reaction gas conversion efficiencies withoutany significant powder or polymeric inclusions.
 2. The process asdefined in claim 1 wherein said reaction gas species are deposited atgreater than 50 Angstroms per second.
 3. The process as defined in claim1 wherein said reaction gas species are deposited in the range of 50 to250 Angstroms per second or greater.
 4. The process as defined in claim1 including evacuating said reaction vessel to a deposition pressure inthe range of 0.001 to 0.1 Torr.
 5. The process as defined in claim 1including converting greater than 10 percent of said reaction gas toreaction gas species.
 6. The process as defined in claim 1 includingconverting substantially 100 percent of said reaction gas to reactiongas species.
 7. The process as defined in claim 1 wherein said reactiongas includes at least silicon.
 8. The process as defined in claim 7wherein said reaction gas is silane (SiH₄).
 9. The process as defined inclaim 7 wherein said reaction gas is silicon tetrafluoride (SiF₄). 10.The process as defined in claim 7 wherein said reaction gas is silicontetrafluoride (SiF₄) and hydrogen (H₂).
 11. The process as defined inclaim 7 wherein said reaction gas is silane (SiH₄) and silicontetrafluoride (SiF₄).
 12. The process as defined in claim 7 wherein saidreaction gas is silane (SiH₄) and germane (GeH₄).
 13. The process asdefined in claim 7 wherein said reaction gas is silicon tetrafluoride(SiF₄) and germane (GeH₄).
 14. The process as defined in claim 7 whereinsaid reaction gas further includes hydrogen (H₂).
 15. The process asdefined in claim 1 wherein said reaction gas includes at leastgermanium.
 16. The process as defined in claim 15 wherein said reactiongas is germane (GeH₄).
 17. The process as defined in claim 15 whereinsaid reaction gas is germanium tetrafluoride (GeF₄).
 18. The process asdefined in claim 15 wherein said reaction gas further includes hydrogen(H₂).
 19. The process as defined in claim 1 wherein said reaction gasincludes a dopant containing compound.
 20. The process as defined inclaim 19 wherein said dopant containing compound includes boron.
 21. Theprocess as defined in claim 20 wherein said dopant containing compoundis diborane (B₂ H₆).
 22. The process as defined in claim 19 wherein saiddopant containing compound includes phosphorus.
 23. The process asdefined in claim 22 wherein said dopant containing compound is phosphine(PH₃).
 24. The process as defined in claim 1 further including the stepof introducing a plasma sustaining gas into said vessel with saidreaction gas.
 25. The process as defined in claim 24 wherein said plasmasustaining gas is argon.
 26. The process as defined in claim 1 whereinsaid deposited semiconductor film has a band gap and wherein saidreaction gas includes a band gap adjusting element.
 27. The process asdefined in claim 26 wherein said band gap adjusting element is a bandgap increasing element.
 28. The process as defined in claim 27 whereinsaid band gap increasing element is carbon or nitrogen.
 29. The processas defined in claim 28 wherein said reaction gas includes ammonia gas(NH₃).
 30. The process as defined in claim 28 wherein said reaction gasincludes methane gas (CH₄).
 31. The process as defined in claim 26wherein said band gap adjusting element is a band gap decreasingelement.
 32. The process as defined in claim 31 wherein said band gapdecreasing element is germanium.
 33. The process as defined in claim 1further including the step of maintaining the temperature of saidsubstrate between about 250° Centigrade and 325° Centigrade.
 34. Theprocess as defined in claim 1 further including the step of adjustingthe power output of said microwave energy to provide power densitiesbetween about 0.1 to 1.2 watts per cubic centimeter.
 35. The process asdefined in claim 1 wherein the frequency of said microwave energy is2.45 Gigahertz.
 36. The process as defined in claim 1 wherein saidprocess forms one step in a multi-step process for forming successivelydeposited alloy layers of opposite (p and n) conductivity type, then-type layer being formed by introducing into said vessel a reaction gascontaining an n-type dopant element which is deposited with thedeposited layer to produce an n-type layer and the p-type layer beingformed by introducing into said vessel a reaction gas containing ap-type dopant element which is deposited with the deposited layer toproduce a p-type layer.
 37. The process as defined in claim 36 whereinthere is deposited between said p and n doped layers an intrinsicamorphous semiconductor alloy layer without a p or n dopant elementpresent therein.
 38. The process as defined in claim 1 wherein saidsemiconductor alloy film is a wide band gap alloy.
 39. The process asdefined in claim 38 wherein said reaction gas is silane (SiH₄) andnitrogen (N₂) or ammonia (NH₃).
 40. The process as defined in claim 39wherein said reaction gas is silicon tetrafluoride (SiF₄) and nitrogen(N₂) or ammonia (NH₃).
 41. The process as defined in claim 38 whereinsaid reaction gas is silane (SiH₄) and oxygen (O₂).
 42. The process asdefined in claim 41 wherein said reaction gas is silicon tetrafluoride(SiF₄) and oxygen (O₂).